Role of surface defects in activation of O2 and N2O on ZrO2 and yttrium-stabilized ZrO2.

The relationship between the structure of both yttrium-stabilized zirconia (YSZ) and ZrO2 catalysts and their ability to activate N2O and O2 is studied by determination of catalytic properties and characterization with TPD, SEM, and XRD. Furthermore, the role of oxygen species formed via dissociation of either O2 or N2O in catalytic partial oxidation of methane (CPOM) is determined. N2O can be activated at both structural defects (e.g., Zr cations located at corners) and intrinsic oxygen vacancies (Zr'(Zr)-V(O)**Zr'(Zr)) and forms two types of oxygen species (alpha-O and beta-O) on the surface, respectively. In contrast, molecular oxygen gives rise to only one type of oxygen species (beta-O), that is, surface lattice oxygen. This type of oxygen species can be extracted by reaction with methane, forming the intrinsic oxygen vacancies again during CPOM. However, the structural defects are not active for oxygen activation during CPOM. Doping ZrO2 with Y2O3 significantly decreases the number of structural defects via replacement of Zr4+ cations by Y3+ cations, located at corners, steps, kinks, and edges of the crystallites. Calcination at higher temperatures results in less structural defects due to both increasing crystallite size as well as transformation to more regular shaped crystallites. High temperature calcinations also increase the activity of YSZ in CPOM. This is attributed to the increase in the exposition of low index planes, especially those (111) with the lowest surface energy and the highest coordination numbers, induced by the thermal treatment.